The present invention relates to a flame atomic absorption spectrophotometer of the type which requires air as a start-up and shut-down oxidant and uses nitrous oxide as a high energy oxidant for the burner flame. The present invention particularly relates to an improved apparatus for safely changing from air as the oxidant to nitrous oxide as the oxidant, and back again.
In atomic absorption spectroscopy, the measurement of the absorption of a radiation beam at a characteristic resonant spectral line for a particular element yields a measure of the concentration of that element in an original sample solution. Presently, one of the most common techniques for atomizing an element for purposes of the absorption measurement is by introducing a liquid sample solution of the element of interest into a gas burner wherein droplets of the solution are vaporized and the elements ultimately atomized, so as to form in the path of the apparatus radiation beam, a substantial quantity of the element of interest in its atomic state. A sample light beam, which originates from a line-emitting light source, and which includes a resonance line of the element to be measured, is directed through the flame. The desired element in the sample absorbs the resonance lines characteristic of the element and the emerging light beam is directed to a monochromator and thence to a detector which measures the degree to which the desired element absorbs the resonance lines of the sample beam. This absorption degree represents the amount of desired element in the sample substance.
In such spectrophotometers, in order to produce a flame which has a high enough temperature for the best measurement results for certain elements, it is preferred to use acetylene gas as a fuel and to use nitrous oxide (N.sub.2 O) as the source of oxygen for the combustion of the acetylene gas. In order to initiate combustion in a safe manner, it is necessary to begin combustion of the acetylene gas using air as the oxygen source, and to then switch over to the nitrous oxide after the acetylene gas flame is ignited and stable.
The essential gases for the steady-state high temperature operation of the burner of a flame atomic absorption spectrophotometer, as described above, are the fuel acetylene and the oxidant nitrous oxide. It is important that both of these essential gases be available in adequate quantities for the system to successfully shift from operation on air to operation on nitrous oxide during start-up, and that those essential gases continue in adequate supply in order to sustain operation on nitrous oxide.
It is also important that there must be a flame at the time of switch-over from air to nitrous oxide, and that the flame be maintained in order to support operation on nitrous oxide. Furthermore, since the entire control system of the spectrophotometer is usually operated by electrical power, including the control of fuel flow, it is important that electric power should continue to be available during operation under nitrous oxide. Still further, it is important that a sufficient volume of oxidant, under sufficient pressure, should be available at the burner of the flame atomic absorption spectrophotometer (not having been reduced too much by the manual flow control adjustment valve) in order to provide for a successful change-over from air to nitrous oxide.
In prior burner control systems, the operator has been relied upon for assuring that some or all of the above mentioned conditions were met in switching from air as the start up oxidant to nitrous oxide as the running oxidant and for assuring that the conditions are maintained. Such an arrangement involves risks because of possible operator error or inattention, and the result can often be an explosion, or improper combustion. Furthermore, some of the above conditions are not immediately apparent to the operator.
Accordingly, it is an important object of the present invention to provide an improved burner control system for a flame atomic absorption spectrophotometer apparatus in which one or more of the above listed requirements for safe switching from air to nitrous oxide as the oxidant are automatically assured, and in which the switch over to nitrous oxide cannot be made without the fulfillment of the condition or conditions.
One prior arrangement for inexpensive burner systems involves the use of a separate valve for each oxidant source, the change over being accomplished by simply closing the valve for one oxidant while opening the valve for the other oxidant. Such a procedure involves the risk that the change over may be slow, resulting in undesirable fuel-oxidant mixtures, or that the oxidant from one source may be completely shut off before the oxidant from the other source is turned on. This may result in extinguishment of the flame together with the risk of possible later explosion if the change is being made to nitrous oxide.
Another object of the invention is to provide an improved burner system which includes an improved means for switching over from one oxidant to the other in a very rapid manner so as to avoid the possibility of improper fuel-oxidant mixtures.
Another object of the invention is to provide an improved burner system for a flame atomic absorption spectrophotometer which is operable to switch from one oxidant to another while avoiding any risk of an interval with both oxidants shut off.
It is another object of the invention to provide an improved burner system for a flame atomic absorption spectrophotometer apparatus which automatically fulfills one or more of the above mentioned requirements for safe operation and which is very efficient and cost effective.
Other objects and advantages of the invention will be apparent from the following description and the accompanying drawings.